Mineralogy and Petrology最新文献

Seven experiments exploring the reaction of titanite with various hydrothermal solutions have been carried out at 700 °C and 200 MPa for a run duration of 16 days. In experiments involving fluids consisting of NaCl+H₂O, KCl+H₂O, CaCl₂+H₂O, 2M NaOH, or 2M KOH, no reaction of the titanite with the fluid was observed other than a slight dissolution of the titanite. Experiments involving NaF+H₂O and Ca(OH)₂+H₂O resulted in visible alteration of the titanite in texture and composition, coupled with the formation of perovskite. In the NaF+H₂O experiment, perovskite, enriched with rare earth elements (REE), formed as euhedral to subhedral crystals on the surface of the recrystallized titanite. In the Ca(OH)₂+H₂O experiment perovskite took in minor amounts of REE, and formed as a reaction rim partially replacing the titanite via a coupled dissolution-reprecipitation reaction. Wollastonite, along with minor calcite, and grossular garnet, formed as an outer rim on the perovskite. In the NaF+H₂O experiment major and trace elements were leached from the titanite, whereas in the Ca(OH)₂+H₂O experiment no leaching of major or trace elements was observed. Nb/Ta, Th/U, and Y/Ho were investigated as potential indicators of hydrothermal processes. While the Nb/Ta ratio was altered in the experimentally metasomatised titanite, the degree of alteration was the same for both fluids. In contrast, only small changes in the Th/U and Y/Ho ratios between the altered and original titanite were seen for either experiment. The formation of perovskite at the expense of titanite in NaF+H₂O or Ca(OH)₂+H₂O fluids demonstrates how titanite reacts with these fluids in simple, low silica activity systems under mid to upper crustal P-T conditions.

The Eocene Rasht-Abad volcanic rocks are located in the Alborz-Azerbaijan magmatic belt (including the Tarom-Hashtjin province) of NW Iran. Those are mainly mafic to intermediate with calc-alkaline affinities, comprising andesite, andesite-basalt, trachy-andesite, and dacite. Clinopyroxene ranging in composition from diopside to augite is the most significant mafic mineral of the basic rocks. Aluminum partitioning between tetrahedral and octahedral sites shows that those crystalized at low pressure. Ferric iron of clinopyroxene also indicates high oxygen fugacity for formation of crystal. Geothermobarometry using clinopyroxene-melt equilibrium calculations constrains the crystallization temperature and pressure of this mineral as 1100–1200 °C and 2–6 kbar. Ce/Pb values of the mafic lavas are lower than values expected for mantle-derived melts but do not support crustal contamination. Co-existing basalt to trachyandesite lavas display parallel and tight REE patterns that suggest these rocks originated from a common mantle source or parental magma. Tectonomagmatic discrimination diagrams and mineral compositions are consistent with the host volcanic rocks having the characteristics of continental margin arcs. Geochemical data are consistent with the parental magma of mafic-intermediate rocks of Rasht-Abad area being derived from a typical subcontinental lithospheric mantle which was enriched by subducted slab-derived fluids and melts during tectonic events in the active continental margin. The data support a model of Eocene flare-up magmatism associated with rollback of a flattened slab.

In this study, laser ablation–multicollector–inductively coupled plasma–mass spectrometry (LA–MC–ICP–MS) of apatites and LA–ICP–MS of zircons are used to collect U–Pb geochronological data, Rb–Sr isotope chronology is used to analyze alkaline feldspar and plagioclase, and archival apatite geochemistry data for the exposed Naitoushan basalt and Heishigou dike in the Changbaishan Tianchi volcano (CTV) are accessed to examine the petrogenesis and determine the origin of basaltic magmatism in the CTV. The Naitoushan basalt and Heishigou dike formed at 22.2–18.7 and 0.230–0.218 Ma, respectively. In situ oxides, volatiles, trace element geochemistry and Sr–Nd isotopes of apatite are reported for two samples. Most apatites are in the early crystallization phase and form inclusions in plagioclase that are euhedral or subhedral. They have higher MgO and K₂O/Na₂O concentrations; lower F and Cl concentrations; Ba, Sr, Nb, Ta, Zr, Hf, K, and Ti depletion; and Th, U, Ce, Pb, P, and Nd enrichment. All apatite samples are enriched in light rare earth elements (REEs) relative to heavy REEs and have relatively homogeneous Th/U, Zr/Hf, La/Sm, and Nd/Tb ratios and Sr–Nd isotopic compositions; thus, their host magmas potentially have the same magmatic origin as oceanic island basalt. The apatite La, Yb, and U contents, Eu/Eu* and La/Yb values, and high REE contents show a weak crystallization sequence in the mafic magma. This study demonstrates that the pre-shield and post-shield mafic magmas in the CTV were likely derived from an enriched mantle source with an I-type signature related to the rollback of the Pacific plate.

The Murun massif (Aldan shield, Russia) is particularly intriguing due to its group of alkali Ca-(K)-(Na) silicates, which includes many new and rare species. Additionally, it is also of interest for its beryllium mineralization. One of the unique beryllium Ca-(K)-(Na) silicates, odintsovite K₂Na₄Ca₃Ti₂Be₄Si₁₂O₃₈, was investigated in this study. The aim of this study is to provide new insights into the crystal chemistry and spectroscopic properties of odintsovite. DFT modelling was used to interpret the experimental IR and Raman spectra. Determining the crystal-chemical formula of odintsovite is challenging due to the presence of atoms in different structural positions with varying valence states. The distribution of cations was determined by combining electron probe microanalysis with the results of crystal structure refinement. Luminescence of Eu³⁺ was observed in odintsovite upon excitation at around 532 nm. Analysis of the luminescence band splitting is related to the 4f–4f transitions in Eu³⁺ ions. Additionally, upon excitation at around 370 nm, luminescence with a peak at around 410 nm, associated with 5d–4f transitions in Ce³⁺ ions, was observed.

This paper reports the metamorphic texture of cordierite megacrysts and the metamorphic P–T path of a newly exposed section of gneiss in East Antarctica. We used mineral textures and pseudosection modeling to reconstruct the metamorphic P–T path of cordierite- and spinel–garnet-bearing gneisses from Botnnuten, an isolated nunatak located ~ 60 km from the southern edge of Lützow-Holm Bay in East Antarctica. The gneisses underwent low-P granulite-facies metamorphism at 5.0–6.1 kbar and 850 ± 20 °C followed by isobaric cooling. The isobaric cooling path implies long residence in the middle to shallow crustal level without rapid exhumation. This contrasts with the widely recognized clockwise P–T path of basement rocks of the Lützow-Holm Complex. The rocks at Botnnuten have long been considered part of the Lützow-Holm Complex based on their petrographical features and geothermobarometric data. However, the present results, combined with a reevaluation of available data, indicate the metamorphic history of the Botnnuten gneisses is more comparable to that of the Yamato Mountains, located southwest of the study area.

This study presents and discusses first detailed petrographic, microthermometric and Raman spectroscopic data from quartz-hosted fluid inclusions at Rudnik Zn-Pb-Cu-Ag skarn deposit (Serbia) and combines them with the information on skarn- and ore paragenesis. Three periods in the metamorphic-hydrothermal history of the deposit are recognized: 1) the pre-ore prograde skarn period when garnet-clinopyroxene skarns formed, 2) the syn-ore period that encompasses a retrograde stage marked by epidote and zoisite and a quartz-sulfide stage characterized by quartz, pyrrhotite, sphalerite, galena and chalcopyrite, and 3) the post-ore period associated with precipitation of calcite and quartz. The hydrothermal evolution is inferred from studying six groups of quartz-hosted fluid inclusions (FI). Two-phase FI of high- (Group A) and moderate salinity (Group B) are found in quartz cores and homogenized at 380–390 °C (mode) and 370–380 °C (mode), respectively. Group A FI consists of H₂O-NaCl liquids and CO₂-CH₄ gas mixtures and likely represents the original fluid composition, whereas Group B FI records dilution of the original fluid at constant temperature, with a slight increase in CH₄ contents. The quartz cores also contain Group C as volatile-rich FI (mostly CO₂ with up to 10 mol% CH₄ and H₂S) of a moderately low salinity and liquid-rich Group D FI composed of pure water with homogenization temperatures of 180–200 °C (mode). The transitional zones of quartz crystals show overgrowth textures and host Group E FI with low salinity that homogenized at 235–401 °C, which vapour phase is a CO₂-CH₄ mixture with up to 17 mol% CH₄. Group F comprises FI found within the rim zones of quartz crystals and they exhibit a low salinity and homogenization temperatures between 259–365 °C. Accordingly, the hydrothermal history at Rudnik involved: a) mixing of different salinity fluids at high temperatures (Groups A and B—retrograde stage), b) introduction of fluids with high volatile contents (Group C) and cooling of fluids with constant salinity (between Groups E and F), which likely correspond to the quartz-sulfide stage, and c) inflow of meteoric water (Group D—the post-ore quartz-calcite stage).

Natural corundum shows two types of twins: “basal twin”, by reflection on (0003) pinacoid, very rare, and “rhombohedral twin”, by reflection on ((10overline{1 }1)), more frequent. The analysis of the structural continuity across the composition plane does not show any reason for a large difference in occurrence frequency, which is likely related to the limited development of the (0003) plane in the characteristic morphology of corundum. “Basal twins” occur with unusually high frequency in samples from Greenland, which also present an atypical platy morphology, where the (0003) face is well developed. This observation seems to confirm a morphological control on the occurrence of the “basal twin”. All analysed twinned samples show macrosteps on their pinacoidal faces and this feature has been related to the high-temperature conditions and intense fluid-rock interactions of Greenland deposit. This clearly suggests a strong relationship between the “basal twin” occurrence, the development of basal faces, and the formation conditions. However, due to the complex geological context and the different features of samples (e.g. two individuals with almost the same size versus several lamellae stacked along c axis), it is not possible to establish with certitude if the “basal twins” observed in Greenland samples are growth or mechanical twins.

Rapakivi granite is characterized by its unique structure, which has important implications for tectonic settings, magmatic processes, and crust–mantle interactions. In this study, we conducted a combined analysis of the petrography, mineral chemistry, geochemistry, and zircon U–Pb dating and Lu–Hf isotopic compositions of the Niujiaoshan Early Paleozoic rapakivi-textured granite from the North Qinling Belt. Zircon U–Pb dating yielded a crystallization age of 447 ± 7 Ma, which is younger than the ultra-high-pressure (UHP) metamorphic age (~ 500 Ma) but similar to the granulite facies retrograde age (~ 450 Ma) of UHP eclogites and felsic gneisses in the North Qinling Belt. The rapakivi feldspar phenocrysts have ovoid K-feldspar cores, which are rich in mineral inclusions, such as amphibole, biotite, quartz, and plagioclase, indicating early crystallization. The ovoid K-feldspar cores are mantled by oligoclase, whreras the matrix comprises biotite, amphibole, and coarse-grained plagioclase. The amphibole and biotite in the granite are rich in Mg and are indicative of a crust–mantle origin. The ε_Hf (t) values of the zircons range from − 2.04 to + 3.63, suggesting formation via crust–mantle interactions. The rapakivi-textured granite displays high-K meta-aluminous I-type granite affinity, with high SiO₂, K₂O, and Na₂O contents. Based on the geological background and results of this study, we propose that the Niujiaoshan rapakivi-textured granite was formed via the mixing of crustal materials induced by upper mantle magma during the exhumation of the North Qinling UHP metamorphic terrane, which occurred in a post-orogenic setting.

Occurrence of “ferrian chromites” have earlier been reported from the Mesoarchean chromite deposits in the Boula-Nuasahi ultramafic complex (BNUC) of India. We have investigated the chromitite bodies in the southern part of the BNUC (i.e., Bangur area) with respect to the mode of occurrence, petrography, chemistry and structure of the chromite types. Although morphologically five varieties of chromite ore were found, chemically only three types of chromite can be distinguished based on EPMA analysis. These are: 1) Type I: magnesiochromite [high Cr₂O₃ (57–65 wt.%), low iron (FeO: 13–17 wt.%), X^Fe3+: < 0.1 apfu]; 2) Type II: ferrian chromite [moderate Cr₂O₃ (43–53 wt.%), high iron (FeO: 27–30 wt.%), X^Fe3+: 0.1 to 0.5 apfu]; and 3) Type III: ferrichromite [low Cr₂O₃ (19–29 wt.%), very high iron (FeO: 55–67 wt.%), X^Fe3+: 0.5 to 1.0 apfu]. Stoichiometrically calculated Fe₂O₃ content is very high in some grains (maximum 47 wt.%). Geochemical discrimination diagrams for the Type I pristine magnesiochromite suggest a dominantly boninitic parental magma. Trace element data obtained from LA-ICP-MS indicate that the Type II chromite has formed from a more evolved magma and is richer in trace elements such as V, Mn, Co, Cu, Pb, Ga, and Nb whereas the Type III ferrichromite shows unusually high Ti and erratic high concentrations of trace elements. Alterations in chromite is noticed in two different thermal regimes: 1) 100–200 °C related to serpentinization of dunite and peridotite rocks where chromite grains show an unaltered core, an intermediate ferrian chromite rim and an outer magnetite rim; 2) 500–600 °C where the entire chromite grain is converted to ferrichromite which can be linked to later intrusion of the Bangur gabbro. While HR-TEM study reveals that all three chromite-types have face-centered cubic structure, Raman spectroscopy indicates that there is a gradual transition of the structural state from normal spinel structure (Type I) through Type II to a fully inverse spinel structure in case of ferrichromite.